1. Field of the Invention
The present invention relates generally to probe cards and more particularly, to a probe adapted to be used in a probe card for high frequency signal transmission.
2. Description of the Related Art
FIG. 1 shows a cantilever-type probe card 1 according to a design of prior art. According to this design, the cantilever-type probe card 1 comprises a circuit board 10, a plurality of coaxial transmission lines 11 arranged around the outer area of the circuit board 10, a probe holder 12 arranged on the inner area of the circuit board 10, and a plurality of coaxial probes 20. The probe holder 12 comprises an insulated base 121 made of a shock-absorbable electrically insulated material, a grounding body 122 made of a metal material and disposed on the insulated base 121 and electrically connected to grounding potential of the cantilever-type probe card 1, and a plurality of locating members 123 fixedly provided on the grounding body 122 to secure the coaxial probes 20. The probes 20 each comprise a metal pin 21, which has a front part 201 (the part between the respective locating member 123 and the probing tip of the respective metal pin 21) and a rear part 202 (the part between the respective locating member 123 and the circuit board 10), a dielectric layer 22 surrounding the rear part 202, and a metal conducting layer 23 surrounding the dielectric covering 22. The metal conducting layer 23 is contacted with the grounding body 122 so as to be electrically connected to grounding potential. Therefore, the coaxial structure of the rear end of each coaxial probe 20 effectively maintains characteristic impedance during transmission of a high frequency signal.
Because the front part 201 of each coaxial probe 20 is designed to serve as an elastic lever arm for bearing and buffering the reaction force fed back from the probing tip when the probing tip is probing the test sites on a wafer under test, it requires a sufficient ambient space for enabling its movement. Therefore, the front part 201 cannot be designed to have a coaxial structure as the rear part 202, i.e., the maintenance of characteristic impedance during high-frequency transmission is limited to the rear parts 202 of the coaxial probes 20, not available at the front parts 201 of the coaxial probes 20. Therefore, the parasitic capacitor induced by the surrounding dielectric environment around each coaxial probe 20 may cause dielectric loss during high-frequency signal transmission.
Further, because the metal pin 21 of each coaxial probe 20 must be surrounded by a certain thickness of dielectric layer 22 to maintain the desired characteristic impedance of the signal transmission, and the installation of the dielectric layer 22 must consider the dielectric loss caused by the parasitic capacitor between the metal pin 21 and the metal conducting layer 23 to avoid the production of a mismatched impedance, the surrounded insulated material must have an optimal thickness varied with its dielectric constant. Whatever insulated material is selected, the diameter of the coaxial probe 20 is much greater than the diameter of the metal pin 21, thereby limiting the arrangement density of the probes 20. Therefore, the aforesaid prior art cantilever-type probe card does not allow for a big number of probes for probing electronic devices with high frequency signaling.
Therefore, it is desirable to provide a cantilever-type probe card that has a high quality of circuit structure for probing highly integrated electronic components rapidly while maintaining the signal quality during transmission of a high frequency test signal for high-precision testing.